To improve drug delivery, lessen toxicity and improve efficacy, various drug delivery systems have been devised. Drug delivery systems have included liposomes which are generally composed of neutral or zwitterionic lipids. The lipids in liposomes arrange themselves into bilayers and entrap one (unilamellar) or more (oligo- or multilamellar) spaces. The spaces between the bilayers of the lipids are usually filled with water. In conventional water filled liposomes, drugs are usually entrapped in the internal aqueous space, although in some cases they may be incorporated in the wall forming materials of the lipid bilayer. In conventional liposomes, it is often difficult to entrap a high concentration of a drug. Relying upon the internal entrapment of the drug, the efficiency necessarily depends upon the volume of fluid outside of the liposomes and circumscribed within the internal aqueous vesicular space. To improve the efficiency of drug entrapment, various techniques, such as ionic or pH gradients, have been employed. Still, the efficiency of drug encapsulation is less than desired. On long term storage, drugs entrapped within liposomes may leak out of the internal aqueous space into the surrounding milieu. The drug may therefore be lost from its desired intra-liposomal location. This is particularly problematic when there is a high concentration of drug entrapped within the liposomes and there is an osmotic gradient across the bilayer membrane. New and better methods of entrapping drugs in liposomes would be beneficial.
Studies have described the effects of calcium and other multivalent cations on membrane asymmetry, lipid distribution, vesicle size, aggregation and fusion. Although the underlying physical causes for the phenomena are debatable, general consensus exists that multivalent cations, such as calcium and magnesium, in the external environment of phospholipid vesicles cause the structures to aggregate into larger, multilamellar structures and promotes fusion. Barium and strontium ions have also been investigated in this regard. Duzgunes et al., Biochemistry, 23:3486-3494 (1984). Species of phospholipids that are particularly pronounced in these effects are the subjet of investigation, as described, for example, by Leckband, et al., Biochemistry 32:1127-1140 (1993), Tilley et al., Biogenic Amines, 5:69-74 (1988) and Kwon, et al., Colloids and Surfaces B, 3:25-30 (1994).
Other areas of investigation focused on the effect of calcium-induced aggregation on phase transition temperature and whether aggregation and fusion phenomena have a temperature dependance. Duzgunes, supra, Kwon, supra, and Tilcock et al., Biochemistry, 23:2696-2703 (1984). The effects of calcium-induced aggregation are so pronounced that efforts have been undertaken to limit the effect in order to control the size of liposomes used in drug delivery systems by forming vesicles in which calcium ions are confined to outer surfaces of the bilayer. European Patent Publication EP 579 703.
Another form of prior art has been the development of polymeric microspheres. Polymeric microspheres may retain the drugs to a better extent than liposomes, but there may be problems with biodegradability and toxicity.
The present invention is directed to, among other things, the development of new and improved drug and contrast media delivery systems that overcome the problems associated with the prior art.